Add a start_date/end_date fields

Also added is a paragraph on dates

Fixes #29

We don't want to specify `"format": "date-time"` because we want to allow
partial date-times. Also, `make check` doesn't check the format anyways.
Add start_date/end_date to BritishColumbiaMosaic as an example
Merge pull request #31 from osmlab/date_range

Date range